1. Field of the Invention
This invention relates to a one-component developing device which develops an electrostatic latent image formed on an electrostatic latent image holder in an electrophotographic copying machine, a printer, or the like, using a one-component developer (hereinafter, sometimes referred to as "toner").
2. Discussion of the Related Art
One-component developing devices disclosed in Japanese Patent Unexamined Publication No. Sho. 47-13088 and Japanese Utility Model Unexamined Publication No. Sho. 53-30339 have the following configuration: As shown in FIG. 5, a one-component developer b supplied from a developer supply roller a or the like is carried on a developer carrier (developing roll) c, and a layer forming member d which is disposed so as to be pressed to the surface of the carrier c contacts with the developer b so that the developer is frictionally electrified and a uniform thin layer of the developer is formed. In a development station A where the developer carrier is in close proximity to an electrostatic latent image holder (photoreceptor drum) e, the developer is selectively transferred to the holder e to develop an electrostatic latent image f. In such one-component developing devices, there arises a problem in that phenomena such as an image hysteresis and a toner scattering from the development station take place. In FIG. 5, g designates a bias power source, p designates a housing of the developing device, and g designates a seal member.
The image hysteresis is a phenomenon in which the developer thin layer is split into two areas, an area which is used in the development of an electrostatic latent image (developing area), and another area which is not used in the development (nondeveloping area), and an image density difference is produced owing to the process of forming the two areas. Specifically, in the developing area, a developer is further supplied onto the carrier and another thin layer is formed by the layer forming member. By contrast, in the nondeveloping area, the same thin layer is passed plural times through the layer forming member without being further supplied with a developer. This produces difference in thickness of a developer thin layer newly formed on the developer carrier (the layer thickness in the developing area is greater by several .mu.m than that in the nondeveloping area), and also in electrification of the developer (the charging amount of the developer in the developing area is smaller by several .mu.C/g than that in the nondeveloping area), whereby the density of an image portion corresponding to the nondeveloping area is made lower than that of an image portion corresponding to the developing area. In other words, a so-called ghost in which the image of the developing area in the previous development process appears like a residual image in the image in the present development process is produced.
The toner scattering phenomenon is caused as follows: The developer is forced to fly by a developing electric field formed in the development station where the developer carrier is in close proximity to the latent image holder. A part of the developer transferred to the holder is not used in the development of an electrostatic latent image on the latent image holder and fails to return to the developer carrier. Such a developer is caused to fall or fly out the developing device by its gravity or an air flow which is produced by the rotation of the developer carrier and the latent image holder, thereby contaminating the interior of the copying machine or the like.
In order to prevent the image hysteresis phenomenon from occurring, conventionally, the following one-component developing devices have been proposed.
For example, a developing device in which as shown in FIG. 6 a developer removing roller i is disposed in the upstream of the developer supply roller i in the developing device housing p so as to be in close proximity to the developer carrier c is proposed (Japanese Patent Examined Publication No. Hei. 1-49945). A voltage having a polarity opposite to that of the developer is applied from a power source h to the developer removing roller i. A nonmagnetic one-component developer remaining on the developer carrier c is electrostatically attracted by the removing roller i to be removed from the carrier. The removed developer is returned to the surface of the developer carrier c. In FIG. 6, j designates a recovery blade for returning the developer on the removing roller i to the carrier c.
Another developing device in which as shown in FIG. 8 a peeling member k for peeling off a developer remaining on the developer carrier c is disposed in the developing device housing p has been proposed by the assignee of the application (Japanese Patent Unexamined Publication No. Sho. 62-251771). The developer which has passed through the development station is peeled from the surface of the carrier c by the peeling member k.
Furthermore, a device in which an electrode roll which rotates in the forward direction without contacting with the developer carrier is disposed in the upstream of the layer forming member in the developing device housing has been proposed (Japanese Patent Unexamined Publication No. Hei. 2-127669). The developer which has passed through the development station and remains on the carrier is removed therefrom by the electrode roll.
However, these developing devices which have been proposed in order to prevent the image hysteresis phenomenon from occurring have problems as described below. In the case where the removing roller i is employed, the one-component developer which has been removed once from the carrier c is returned as it is to the carrier c. When a process of copying an image P shown in FIG. 7(a) is continuously conducted on several tens sheets and thereafter a process of copying another image Q is conducted, the particle diameter of the toner on the portion of the developer carrier which corresponds to the image area x used in the process of copying the image P differs from that of the toner on the portion of the developer carrier which corresponds to the nonimage area y. More specifically, the particle diameter of the toner on the portion of the developer carrier which corresponds to the nonimage area y is somewhat smaller than that of the toner on the portion of the developer carrier which corresponds to the image area x. As a result, there arises a problem in that the image hysteresis appears in which as shown in FIG. 7(b) the density of the image portion corresponding to the nonimage area y is lower than that of the image portion corresponding to the image area x.
In the case where the peeling member k is employed, additives and the like which are added to the developer are not removed from the developer carrier c but caused to adhere thereto by the sliding contact of the peeling member k, thereby contaminating the surface of the developer carrier c. As shown in FIG. 9, the developer immediately after the removal process conducted by the operation of the peeling member k has an impaired rise of the charging property (the charging amount is reduced) as compared with that which has not yet passed through the peeling member k. These produce problems in that fogging easily occurs in the nonimage area (background), and that with the elapse of time the developer itself is deteriorated by the repeated sliding contacts between the developer and the peeling member k.
The inventors experimentally constructed a developing device in which the electrode roll is employed, and conducted experiments using this developing device. As a result, it was noted that, since the electrode roll is disposed inside the device housing, the developer removed by the electrode roll is mixed with the developer stored in the developer storage, thereby producing a problem in that the effect of the image hysteresis depends on the toner amount in the housing.
Any of these proposed developing devices cannot attain the effect of preventing the above-mentioned toner scattering phenomenon from occurring.
As a one-component developing device which can prevent the toner scattering phenomenon from occurring, for example, a developing device in which a toner scattering prevention plate is disposed in the downstream of the development station in the device housing p as shown in FIG. 10 has been proposed (Japanese Patent Unexamined Publication No. Hei. 4-100073).
However, the proposed device has problems in that, although the amount is small, the developer is transported to the outside of the device through a gap between the prevention plate 1 and the electrostatic latent image holder e, by an air flow due to the rotation of the holder e, and that the developer adheres to the front end of the prevention plate l to be accumulated thereon and therefore the accumulated developer is transported to the outside of the device.
As described above, there is no developing device in which both the image hysteresis phenomenon and the toner scattering phenomenon can be prevented from occurring.